The present invention relates to a rubber composition for a breaker cushion and a tire having a breaker cushion using the same. Specifically, the present invention relates to a rubber composition for a breaker cushion capable of suppressing exothermic heat and lowering of physical properties of a rubber, and a tire having a breaker cushion using the same.
A layer called a breaker cushion is provided between the edge portion of the breaker and the case. The breaker cushion largely affects ride quality, durability and the like.
Recently, needs for improving a service life of a tire has been enhanced and improvement of abrasion resistance of a tread rubber and enlargement of a tread width has been also promoted. It is also urgent need to drastically improve fatigue resistance of a breaker cushion rubber more than a conventional one. Consequently, it is necessary to adopt a composition excellent in hardness, elongation at break after aging and the like for the breaker cushion rubber, while keeping steering stability.
In particular, in the case of tires for heavy load such as a bus and an autotruck, temperature rising of the edge portion of a breaker is remarkable, and separation may occasionally generated between an edge rubber of the breaker and steel cords due to thermal aging. Naturally, a breaker cushion portion nearby the area is subject to very intensive thermal fatigue at running. Further, the rubber is remarkably hardened, and the breaker cushion rubber is destroyed by external stimulation, which could cause burst. Accordingly, the requisite properties of the breaker cushion rubber include that it is a rubber which is small in excessive increase of hardness due to exothermic heat and lowering of tensile property after aging while enduring heavy load.
Consequently, for example, when high grade carbon black such as SAF class having favorable tensile property is used, it is deficient in low exothermic property and when low grade carbon black such as FEF class having favorable low exothermic property is used, the tensile property is lowered; therefore, ISAF class and HAF class carbon black have been preferably used. However, since the long life of a tire is promoted in future, further improvement of durability is an urgent need.
JP-A-2004-161862 discloses a rubber composition for a breaker cushion excellent in low exothermic property and small in lowering of tensile property after aging while keeping steering stability, by comprising the specified amount of specific silica and carbon black in the rubber component, but the hardness after aging was not described at all, and further, a retention rate of the tensile property was also not insufficient.